A significant amount of the potent greenhouse gas methane may be leaking into the atmosphere from abandoned oil and gas wells, according to a study in Pennsylvania out Monday.

The study in the Proceedings of the National Academy of Sciences is based on direct measurements of methane outflow from and near 19 abandoned oil and gas wells in the northeastern US state.

Researchers at Princeton University in New Jersey and Stanford University in California took nearly 100 measurements over the course of seven months in 2013 and 2014, and found that all the wells they studied emitted methane, whether they were in forests, wetland, grassland, or river areas.

Three of the 19 wells were so-called "high emitters," sending out methane at three time the median, or midpoint, rate.

The study said that if the 19 wells studied are typical of the overall situation in the northeastern state of Pennsylvania, the abandoned wells may account for four to seven percent of methane emissions in the state.

The study authors urged environmental regulators to take account of abandoned wells when calculating total methane emissions.

Previous research has suggested that methane emissions from approximately three million abandoned oil and gas wells across the United States, which are not accounted for in US government estimates, may be the second largest contributor to total US methane emissions.